SimpleMachine

SimpleMachine

Challenge : SimpleMachine

Description :
(fixed-point challenge)

Classic Check Flag Challenge Machine

DOWNLOAD :
http://ctf.codegate.org/099ef54feeff0c4e7c2e4c7dfd7deb6e/116ea16dbeabe08d1fe8891a27d0f16b

point : 333

Summary

• Reverse Engineering.
• The basic of code virtualization technique.

Tools

• HxD
• IDA 7.0
• gdb-peda

Description

• Notice that simple_machine is ELF file and target is data.
  • 
• Open simple_machine with IDA 7.0, analyze, and generate some pseudo-codes.
  • 
    • pseudo-code of main function.
    • Usage: ./simple_machine target
    • There is a limitation to the length of target.
    • It uses code virtualization technique.
    • We can assume that target is a kind of virtualized code script.
  • 
    • pseudo-code of vm_func function.
    • There is a control function.
  • 
    • pseudo-code of control_func function.
    • Fetch and decode operations from target.
• Based on control_func function, set breakpoint on and debug it.
  • 
    • Run gdb-peda ./simple_machine and b *0x80017ca(Entry point of control_func).
    • Debug and understand how the program works.
  • 
    • First operation is getting input from stdin.
    • 6 ADD-CHECK chain operations are checking if first 12 characters of input are CODEGATE2020.
    • Next 32 operations are generating 12 hashes, calculating XOR with input, and checking if is zero.
    • if all check is passed, print GOOD! to stdout.
    • Last operation is just exiting the program.
• Write and execute python script to get flag.
  • 
  • get_flag.py
• CODEGATE2020{ezpz_but_1t_1s_pr3t3xt}